An electromagnetic hearing aid receiver or other comparable electromagnetic acoustic transducer inherently generates a magnetic field; without shielding, a substantial portion of that field is radiated externally of the transducer. This external magnetic field will induce spurious signals in any other electromagnetic device in the immediate vicinity. The external magnetic field around an electromagnetic hearing aid transducer frequently creates spurious feedback signals in a pickup coil employed for coupling the hearing aid to a telephone receiver.
A substantial improvement in containment of the external field of an electromagnetic hearing aid receiver is provided in the transducer construction having a magnetic shield that is described and claimed in Carlson U.S. Pat. No. 3,111,563. Although the self-shielding receiver construction covered by that patent affords appreciable improvement in minimizing the effect of the external field of an electromagnetic hearing aid receiver or like device, it does not solve the problem completely. Thus, most hearing aid receivers and other electromagnetic transducers, particularly miniature devices, continue to present appreciable problems when brought into close proximity with other electromagnetic transducers or couplers, whether microphones or receivers or coupling coils. The present invention is intended to remedy this situation and to provide much better and more effective shielding than has previously been afforded.